What is the Society of St. Pius the X? I have heard it in a rather Dark tone (with thunder rolling and lighting cracking in the Background sort of stuff). Who are they?
They are schismatic Catholics who followed Archbishop Lefebvre into schism. They fell off of the right side of Peter’s barque, so to speak.
Many of them deny the validity of the NO Mass.
Others here can give you a better idea on who they are than I can.